1. Field of the Invention
The present invention relates to an extrusion press and an extrusion control method and, more particularly, to an extrusion press and an extrusion control method capable of improving the product yields by controlling a container sealing force that acts on the end surface of a die to be constant during the entire process of extrusion.
2. Description of the Related Art
A conventional extrusion press comprises an end platen linked to a tie rod and a main cylinder device and on the side of the end platen, a container is arranged, in which a billet is loaded, with a die being sandwiched in between, and on the side of the main cylinder device, a stem is provided to a crosshead to be driven integrally with a main ram that enters/exits from the main cylinder. Then, the billet loaded in the container is extruded under pressure by the stem using the extrusion force by the main cylinder device and thus a predetermined product is extruded and molded from a die.
In such an extrusion press, it is desirable that the container sealing force be constant during the entire extrusion process, however, the billet in the container gradually becomes shorter and shorter in the extrusion process, and therefore, normally the force required for extrusion when extrusion starts is larger than that when extrusion ends. That is, even if the extrusion resistance of a die (required extrusion force that acts on a die) is constant, the frictional resistance between the container inner wall and the billet becomes smaller as the length of the billet is reduced, and therefore, the extrusion force gradually reduces on the whole.
If the extrusion force changes during the extrusion process, the force that acts on the die of the extrusion press changes, and as a result the amount of deflection of the die is not constant during the extrusion process. Consequently, there is a problem that the product obtained by a conventional extrusion press is not uniform in thickness and shape in the longitudinal direction.
Further, the change in the extrusion force causes the container sealing force against the die to vary and there used to be a problem of the occurrence of a so-called bursting phenomenon, in which the billet bursts forth from a sealed part.
In the extrusion press disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-274821, a pressing means for pressing under pressure a container to the side of a die so as to seal between the container and the die is provided to a crosshead that links a main cylinder device and a main ram. Then, when the length of a billet becomes equal to or less than a predetermined length during extrusion process, the container is pressed under pressure with the pressing means and a container seal force is applied between the container and the die by the pressing pressure, and thereby, the burst of the billet is avoided (patent document 1).
However, with the technique disclosed in above-described patent document 1, the container sealing force applied to the end platen via the die is added to cause the extrusion force to act and it is possible to keep constant the displacement of the end platen and the die in the longitudinal direction of extrusion of a product and to obtain a uniform product, however, during the extrusion process, the container is pressed under pressure by the pressing means, and therefore, there is a problem that a maximum load pressure when extrusion starts acts on the main cylinder device and the amount of energy consumption increases during extrusion process.    [Patent document 1] Japanese Unexamined Patent Publication (Kokai) No. 4-274821.